This invention relates to composite materials and more particularly to tough, fiber-reinforced composites having good resistance to moisture. Still more particularly, this invention relates to fiber-reinforced composites comprising amide-imide copolymer matrix resins that exhibit improved water absorption characteristics and retention of mechanical properties after exposure to humid environments.
Fiber-reinforced composites are high strength, high modulus materials which are finding wide acceptance for use in sporting goods and in producing consumer items such as appliances. Such materials are also finding increased acceptability for use as structural components in automotive applications, as components of buildings and in aircraft. Typically, the composites used in structural applications comprise structural fibers in the form of continuous filaments or woven cloth embedded in a thermosetting or thermoplastic matrix. Such composites may exhibit considerable strength and stiffness, and the potential for obtaining significant weight savings makes them highly attractive for use as a metal replacement. However, acceptance of many composite materials for use in extreme environments has been limited by their tendency toward loss in important mechanical properties. This characteristic is particularly noted in composites comprising matrix resin based on condensation polymers such as polyesters, polyimides, polyamides or the like. Such resins tend to absorb moisture, becoming plasticized and losing in rigidity and thermoproperties such as Tg. In addition, where the linking groups are sensitive to hydrolysis, some loss in properties may result by way of hydrolytic degradation of the matrix resin.
The moisture sensitivity of such composites and the resulting inability of such composites to retain useful rigidity, toughness and high temperature properties has been recognized as being a serious problem for many years. The composites industry has long been involved in finding ways to overcome these deficiencies, including through the development and use of thermosetting resins, which tend to be less sensitive to moisture. Considerable effort has also been expended over the past two decades directed toward improving the moisture sensitivity of well-known condensation polymers that have otherwise highly desirable characteristics for use as matrix resins.
Polyamide-imides are condensation polymers finding use in a wide range of applications such as adhesives, molding compositions, fibers, films, composites, laminates, etc., owing to a desirable combination of properties. Torlon.RTM. polyamide-imides, available from Amoco Performance Products, Inc., are examples of commercial polyamide-imides.
Despite the many desirable properties of polyamide-imides, their utility in certain applications has been limited by a tendency to absorb water, leading to loss of mechanical properties on exposure to high temperatures. So-called annealing or post-curing treatments of polyamide-imide or polyamide-amic acid fabricated parts such as are disclosed in commonly assigned U.S. Pat. No. 4,167,620 allow water liberated due to imidization and chain extension reactions as well as absorbed moisture to diffuse out of fabricated parts and may improve retention of mechanical properties. However, since polyamide-imide resins tend to absorb water when exposed to humid environments, these treatments are not sufficient to permit the use of currently available, commercial polyamide-imides in certain demanding applications. It also is known to add certain metal oxides to polyamide-imides to tie up absorbed moisture as well as water liberated during imidization and chain extension reactions to avoid cracking and sacrifices in physical properties. Again, however, this approach does not yield sufficient improvement of presently available materials or prevent subsequent moisture absorption and further reduction in mechanical properties.
Aromatic polyimides typically do not absorb water to as great an extent as polyamide-imides. However, the utility of aromatic polyimides is limited because of their lack of solubility. Further, their high glass transition temperatures ("Tg") make melt processing impractical or impossible.
U.S. Pat. No. 4,017,459, assigned to the Upjohn Company, discloses amide-imide polymers and copolymers prepared from 2,2-bis(4-(p-aminophenoxy)phenyl) propane and trimellitic anhydride halide or from 2,2-bis(4-(p-isocyanatophenoxy)-phenyl) propane and trimellitic acid or anhydride. According to the patent, such polyamide-imides are melt processable, such as by injection molding, and useful in manufacture of articles such as gears, ratchets, clutch linings, bearings, pistons and cams and electrical components. In contrast, the patentee teaches that polyamides prepared from the above-named diamine and isophthalic acid and polyimides prepared from that diamine and pyromellitic acid dianhydride or benzophenone tetracarboxylic acid dianhydride are intractable in the sense of lacking sufficient solubility for solution processing, lacking in melt processability or lacking both.
U.S. Pat. Nos. 4,111,906 and 4,203,922, both assigned to TRW, Inc., disclose that although processability of polyimides can be improved by using the same in predominantly polyamide-amic acid form and imidizing during a final fabrication step, such an approach is disadvantageous because voids in the final products result from water liberated due to the imidization reaction. These patents also state that chemical and thermal stability are improved by preparing polyimides from 2,2-bis(4-(p-aminophenoxy)phenyl) hexafluoropropane. According to the '906 patent, polyimides prepared from this diamine and a dianhydride are useful as coatings, adhesives and as a matrix for laminated glass or graphite structures. Polyimide foams prepared from pyromellitic acid dianhydride or other aromatic tetracarboxylic acid dianhydrides and such diamine in combination with a second aromatic diamine are disclosed in U.S. Pat. No. 4,535,101, assigned to Imi-Tech Corporation. Preparation of polyamides from the above-named diamine and diacids also is disclosed in the '906 patent. The abstracts of both the '906 and '922 patents mention polyamide-imides; however, no additional information is provided.
U.S. Pat. No. 4,340,697, assigned to Toray Industries, Inc., discloses melt processing difficulties with polyamide-imides and purports to remedy the same by blending with polyphenylene sulfide, polyamide, aromatic polyester, polyphenylene ether or a phenoxy resin. According to this patent, polyamide-imides may contain, in addition to a repeating, main structural amide-imide unit, up to 50 mole percent amide or imide units, the latter being introduced into the polymer by replacing a portion of the aromatic tricarboxylic acid component with pyromellitic acid dianhydride or benzophenone tetracarboxylic acid dianhydride.
U.S. Pat. No. 4,599,383, assigned to NTN-Rulon Industries Co., Ltd., discloses compositions having improved water absorption properties containing a polyamide-imide resin in combination with a polyetherimide and a fluoro resin component.
U.S. Pat. No. 4,755,585, assigned to M & T Chemicals, Inc., discloses polyimides, polyamide acids, polyamide-imides, polyesterimides and polyesteramides containing at least 10 mole percent of a reaction product of an aromatic or aliphatic mono- or dianhydride and certain aromatic diamines having an unsubstituted or halogen- or hydrocarbyl-substituted, paraphenylene radical linked by like or different alkylene, alkenylene, sulfide or oxy groups to two unsubstituted or halogen- or hydrocarbyl-substituted, monovalent, aminophenyl radicals provided that the linking groups are not contemporaneously both sulfide or oxy. Such products are said to exhibit improved processing characteristics and thermal stability and to have utility in widespread applications. Two polyamide-imides and films thereof are demonstrated in the examples. Numerous anhydrides, dianhydrides and diamines are named in this publication and mixtures of anhydrides and dianhydrides are mentioned. It is also reported that mixtures of the above-described diamines with other diamines may be used. Interestingly, such other diamines are said to include 2,2-bis(4-(p-aminophenoxy)phenyl) propane and sulfone, although the publication also mentions, with supporting citations, that polyimides prepared from such diamines and dianhydrides are insoluble and that polyamide-imides prepared from such diamines are of uncertain solubility and processability.
Although a considerable effort has been expended to develop improved composites comprising polyamide-imide matrix resin, most of the presently available composite materials based on such resins lack the combination of good processability and reduced moisture sensitivity needed for use in demanding environments. Composite materials and filled molding compounds based on such resins are needed for use in a variety of applications, including sporting goods and under-the-hood automotive applications. In addition, where the compressive strength of composites based on such resins is not reduced or, more preferably, is improved, the composites would find wide acceptance for use in producing structural components for sporting goods, automobiles, buildings and aircraft.